The present invention relates to a PSK (phase shift keying) system and method in a coherent optical fiber transmission and more particularly to a PSK system and method adapted to be able to directly modulate an injection current supplied to a laser diode.
Of the optical fiber transmission systems in practical use today, an IM/DD (intensity modulation/direct detection) system in which an intensity modulated light beam is directly received by a photodetector and converted thereby into an electric signal is quite general. In recent years, however, research and development for a coherent optical fiber transmission system has become actively carried on because of strong demands for larger transmission capacity and longer transmission distance. According to this system, since a coherent light beam from a laser diode is used as the carrier and its frequency, phase, etc. are modulated on the transmission side and the received light beam is mixed with a local light beam so as to be subjected to heterodyne detection or homodyne detection on the reception side, a greater improvement in the reception sensitivity compared with the IM/DD system can be achieved. Further, after the detection of the light has been performed, i.e., after the light signal has been converted into an electric signal, frequency selection can be made rather easily. Hence, high-density frequency-division multiplexing can be achieved by this system and the transmission capacity by a single optical transmission path can thereby be greatly increased.
As a system or method for transmitting information with the information carried by the wave parameter of a light beam emitted from a laser diode and being suitable for high speed transmission, DPSK (differential phase shift keying) or CPFSK (continuous phase frequency shift keying) has hitherto been known.
In the DPSK system, in order that a demodulation by delayed detection by one bit is performed on the reception side, differential coding is made on the transmission side in advance. The modulation through the differentially coded signal is an indirect modulation using an external modulator.
In the CPFSK system, on the other hand, the oscillation frequency of the laser diode is directly modulated on the transmission side so that the phase deviation between different signs becomes over .pi. with the phase maintained continuous, and a demodulation with delayed detection is performed on the reception side. The delay time is set according to one time slot and the modulation index.
In the DPSK system, an external modulator is required and the loss due to its insertion (for example, 2 to 4 dB) becomes a cause of the deterioration in the reception sensitivity. Further, since most of the external modulators are such that utilize the electro-optic effect of an anisotropic crystal, it requires driving voltage of several to ten-odd Volts for obtaining a frequency band of several GHz. Further, a differential coding circuit is required for achieving a demodulation by delayed detection by one bit. Thus, the DPSK system becomes complex in structure.
In the CPFSK system, although neither external modulator nor differential coding circuit is required, the system is liable to be adversely affected by the wavelength dispersion and, hence, its transmission span is limited. Further, the carrier is unable to be reproduced for use in the CPFSK system.